Re: [PATCH v1 1/2] oslib-posix: refactor memory prealloc threads

[David Hildenbrand <david@redhat.com>]

On 08.01.24 19:40, Mark Kanda wrote:
> On 1/8/24 9:40 AM, David Hildenbrand wrote:
>> On 08.01.24 16:10, Mark Kanda wrote:
>>> Refactor the memory prealloc threads support:
>>> - Make memset context a global qlist
>>> - Move the memset thread join/cleanup code to a separate routine
>>>
>>> This is functionally equivalent and facilitates multiple memset contexts
>>> (used in a subsequent patch).
>>>
>>> Signed-off-by: Mark Kanda <mark.kanda@oracle.com>
>>> ---
>>>    util/oslib-posix.c | 104 +++++++++++++++++++++++++++++----------------
>>>    1 file changed, 68 insertions(+), 36 deletions(-)
>>>
>>> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>>> index e86fd64e09..293297ac6c 100644
>>> --- a/util/oslib-posix.c
>>> +++ b/util/oslib-posix.c
>>> @@ -63,11 +63,15 @@
>>>      struct MemsetThread;
>>>    +static QLIST_HEAD(, MemsetContext) memset_contexts =
>>> +    QLIST_HEAD_INITIALIZER(memset_contexts);
>>> +
>>>    typedef struct MemsetContext {
>>>        bool all_threads_created;
>>>        bool any_thread_failed;
>>>        struct MemsetThread *threads;
>>>        int num_threads;
>>> +    QLIST_ENTRY(MemsetContext) next;
>>>    } MemsetContext;
>>>      struct MemsetThread {
>>> @@ -81,7 +85,7 @@ struct MemsetThread {
>>>    typedef struct MemsetThread MemsetThread;
>>>      /* used by sigbus_handler() */
>>> -static MemsetContext *sigbus_memset_context;
>>> +static bool sigbus_memset_context;
>>>    struct sigaction sigbus_oldact;
>>>    static QemuMutex sigbus_mutex;
>>>    @@ -295,13 +299,16 @@ static void sigbus_handler(int signal)
>>>    #endif /* CONFIG_LINUX */
>>>    {
>>>        int i;
>>> +    MemsetContext *context;
>>>          if (sigbus_memset_context) {
>>> -        for (i = 0; i < sigbus_memset_context->num_threads; i++) {
>>> -            MemsetThread *thread = &sigbus_memset_context->threads[i];
>>> +        QLIST_FOREACH(context, &memset_contexts, next) {
>>> +            for (i = 0; i < context->num_threads; i++) {
>>> +                MemsetThread *thread = &context->threads[i];
>>>    -            if (qemu_thread_is_self(&thread->pgthread)) {
>>> -                siglongjmp(thread->env, 1);
>>> +                if (qemu_thread_is_self(&thread->pgthread)) {
>>> +                    siglongjmp(thread->env, 1);
>>> +                }
>>>                }
>>>            }
>>>        }
>>> @@ -417,14 +424,15 @@ static int touch_all_pages(char *area, size_t
>>> hpagesize, size_t numpages,
>>>                               bool use_madv_populate_write)
>>>    {
>>>        static gsize initialized = 0;
>>> -    MemsetContext context = {
>>> -        .num_threads = get_memset_num_threads(hpagesize, numpages,
>>> max_threads),
>>> -    };
>>> +    MemsetContext *context = g_malloc0(sizeof(MemsetContext));
>>>        size_t numpages_per_thread, leftover;
>>>        void *(*touch_fn)(void *);
>>> -    int ret = 0, i = 0;
>>> +    int i = 0;
>>>        char *addr = area;
>>>    +    context->num_threads =
>>> +        get_memset_num_threads(hpagesize, numpages, max_threads);
>>> +
>>>        if (g_once_init_enter(&initialized)) {
>>>            qemu_mutex_init(&page_mutex);
>>>            qemu_cond_init(&page_cond);
>>> @@ -433,7 +441,7 @@ static int touch_all_pages(char *area, size_t
>>> hpagesize, size_t numpages,
>>>          if (use_madv_populate_write) {
>>>            /* Avoid creating a single thread for MADV_POPULATE_WRITE */
>>> -        if (context.num_threads == 1) {
>>> +        if (context->num_threads == 1) {
>>>                if (qemu_madvise(area, hpagesize * numpages,
>>>                                 QEMU_MADV_POPULATE_WRITE)) {
>>>                    return -errno;
>>> @@ -445,49 +453,74 @@ static int touch_all_pages(char *area, size_t
>>> hpagesize, size_t numpages,
>>>            touch_fn = do_touch_pages;
>>>        }
>>>    -    context.threads = g_new0(MemsetThread, context.num_threads);
>>> -    numpages_per_thread = numpages / context.num_threads;
>>> -    leftover = numpages % context.num_threads;
>>> -    for (i = 0; i < context.num_threads; i++) {
>>> -        context.threads[i].addr = addr;
>>> -        context.threads[i].numpages = numpages_per_thread + (i <
>>> leftover);
>>> -        context.threads[i].hpagesize = hpagesize;
>>> -        context.threads[i].context = &context;
>>> +    context->threads = g_new0(MemsetThread, context->num_threads);
>>> +    numpages_per_thread = numpages / context->num_threads;
>>> +    leftover = numpages % context->num_threads;
>>> +    for (i = 0; i < context->num_threads; i++) {
>>> +        context->threads[i].addr = addr;
>>> +        context->threads[i].numpages = numpages_per_thread + (i <
>>> leftover);
>>> +        context->threads[i].hpagesize = hpagesize;
>>> +        context->threads[i].context = context;
>>>            if (tc) {
>>> -            thread_context_create_thread(tc,
>>> &context.threads[i].pgthread,
>>> +            thread_context_create_thread(tc,
>>> &context->threads[i].pgthread,
>>>                                             "touch_pages",
>>> -                                         touch_fn, &context.threads[i],
>>> +                                         touch_fn,
>>> &context->threads[i],
>>> QEMU_THREAD_JOINABLE);
>>>            } else {
>>> - qemu_thread_create(&context.threads[i].pgthread, "touch_pages",
>>> -                               touch_fn, &context.threads[i],
>>> + qemu_thread_create(&context->threads[i].pgthread, "touch_pages",
>>> +                               touch_fn, &context->threads[i],
>>>                                   QEMU_THREAD_JOINABLE);
>>>            }
>>> -        addr += context.threads[i].numpages * hpagesize;
>>> +        addr += context->threads[i].numpages * hpagesize;
>>>        }
>>>    +    QLIST_INSERT_HEAD(&memset_contexts, context, next);
>>> +
>>>        if (!use_madv_populate_write) {
>>> -        sigbus_memset_context = &context;
>>> +        sigbus_memset_context = true;
>>
> Thanks David,
> 
>> Could we just use the sigbus handling alone and support parallel init
>> only when using MADV_POPULATE_WRITE where we don't have to mess with
>> signal handlers?
>>
> 
> Ideally, we're hoping to support this with earlier kernels which don't
> support MADV_POPULATE _WRITE. But, I will check to see if we really need it.

That's around since Linux 5.14, so please try simplifying.

> 
>> Further, how do you changes interact with other callers of
>> qemu_prealloc_mem(), like virtio-mem?
>>
> 
> I'm not familiar with the intricacies of virtio-mem, but the basic idea
> of this series is to *only* allow parallel init during the start up
> phase (while prealloc_init == false). Once we have parsed all the
> command line args, we set prealloc_init = true
> (wait_mem_prealloc_init()) which causes all subsequent calls to
> qemu_prealloc_mem() to perform initialization synchronously. So, I
> *think* this covers the virtio-mem use case. Am I missing something?

Good, so this should likely not affect virtio-mem (which also ends up 
preallocating memory when loading from a vmstate).

To make this all a bit clearer, what about the following to make this:

* Optimize for MADV_POPULATE_WRITE. If we really need support for
   !MADV_POPULATE_WRITE, this is better added on top later.
* Pass in via a parameter that the caller requests async handling. "bool
   async" should be good enough. Then, pass that only from the memory
   backend call, while QEMU is still initializing (we can find a way to
   make that work).
* Provide a function that waits for any remaining async os-mem-prealloc
   activity. That is essentially "wait_mem_prealloc_init", but without
   the special internal flag handling.

Further, I do wonder if we want to make that behavior configurable. For 
example, one might want to initialize backends sequentially using 16 
threads max, instead of consuming multiple times 16 threads 
concurrently. Could either be a machine option or a memory backend 
option (async-prealloc=on). Once hotplugging such backends, we would 
disable it for now, but could allow it in the future.

-- 
Cheers,

David / dhildenb